1. Field of Invention
The present invention is directed to a broach for preparing an intramedullary canal of a bone for receiving the stem component of a prosthesis. Specifically, the present invention is directed to an improved broach designed for optimizing the balance between surgical ease-of-use and minimal gap formation between the prosthesis and the intramedullary canal.
2. Background of Invention
The preparation of the intramedullary canal for receiving a femoral prosthesis is a critical step in cementless hip arthroplasty procedures since the fit between the stem and its associated femur is determined by the size of the prepared cavity. An improper fit of the stem within the cavity often causes microrotation of the stem relative to the femur and unstable fixation.
A major source of early loosening and failure of the prosthesis is misalignment of the prosthesis so that its proximal end is inclined medially with respect to the longitudinal axis of the medullary canal. This misalignment occurs because the broach is implanted in varus, and thus produces a cavity that is not correctly aligned. Implantation of the broach in the neutral position minimizes the possibly of misalignment; however, such implantation requires that the lateral face of the broach cut into the very hard bone of the greater trochanter, which is difficult to penetrate with conventional broaching instruments. Specifically, in the posterior-lateral corner of the implantation site hard bone is often present due to a condensation of cancellous bone which must be machined away to allow the broach to be neutrally aligned. Conventional broach teeth are normally not sufficiently aggressive to remove this bone. In addition, the trochanter is covered with tough tissue which clogs the teeth of the cutting instruments, thus rendering them ineffective in cutting bone.
It has therefore been recognized in the art that in order to remove the strong trochanteric bone, the lateral surface of the broaching instrument must have aggressively cutting teeth. Historically, femora were prepared with bone rasps covered with diamond or hook-shaped teeth. Such bone rasps were relatively easy to use and fairly effective in removing bone, especially if their surface features were coarse. However, such rasps made ragged cavities and were not suitable for applications in which a precise implant/bone fit was important. Moreover, increasing evidence suggests that the presence of gaps at the cementless interface facilitates osteolysis. Consequently, it is essential that the cavity formed within the femur by broaching closely replicates the dimensions of the instrument itself.
In practice, hand broaching is a relatively inaccurate method of machining the proximal femur due to three main factors:
(1) The instrument is not precisely directed. Deviations in the "tool path" of the instrument are reflected in an enlarged cutting envelope leading to gaps at the stem/bone interface. This effect is especially pronounced with aggressive tooth configurations which allow the broach to machine bone wherever the instrument is directed. PA1 (2) The cutting surface erodes bone and does not cut it cleanly. Upon insertion and subsequent extraction, the broach breaks trabeculae and drags them along the interface. This causes a ragged surface which is further enlarged beyond the broach envelope. PA1 (3) The broach acts as a wedge and causes the femur to expand elastically. Upon removal, the implantation site contracts and so may be smaller than the broach if a significant amount of cancellous bone was removed when the bone was in an expanded state.
The foregoing problems are of particular concern when broaching the medial, anterior/medial, and posterior/medial surfaces of the femoral canal, since these are crucial areas for attachment of the femoral stem within the femur. Thus, it is important that these areas be cut accurately and cleanly with very minimal gap formation resulting between the implant and the intramedullary canal.
While there exist several types of broaches, it is believed that none of them teach or suggest the strategic placement of a plurality of teeth having at least two different tooth configurations on a broach to optimize broaching efficiency while at the same time minimizing subsequent gap formation. The vast majority of such broaches comprise a single tooth configuration positioned about the entire surface of the broach, such as those disclosed in U.S. Pat. No. 4,552,136 (Kenna) and U.S. Pat. No. 5,041,118 (Wasilewski), for example. In Kenna, the teeth comprise a single cutting edge and are positioned about the entire surface of the broach with the exception of the distal lateral face and the proximal medial face, which are left substantially blank to avoid gouging of the lateral cortex and medial cortex, respectively. Wasilewski discloses a broach comprising cutting teeth having the same configuration with two pairs of cutting edges, wherein the cutting teeth are positioned on all four faces of the broach. U.S. Pat. No. 5,454,815 (Geisser et. al) is directed to a disposable broach designed for single use and comprises two types of cutting teeth: "course teeth" positioned in the distal, midshaft, and approximately lower third of the proximal regions and "fine" teeth positioned adjacent the course teeth in the proximal region. While Geisser et al. contends that the teeth allow for a "swift and at the same time gentle working of the bone" (col. 2, lines 66-67), there is no teaching as to the strategic placement of teeth to provide for optimal ease of use and cutting efficiency while effecting minimal gap formation, since each of the types of teeth disclosed are positioned on each face of the broach.
Consequently, it is desireable to have a broach that allows for fast and efficient removal of bone, including hard bone that is difficult to remove, while simultaneously minimizing gap formation between the prosthesis and the intramedullary canal, thereby minimizing early loosening and failure of the implant, in particular in procedures wherein a precise implant/bone fit is important.